I have to ask, what are you trying to accomplish? Just a thicker oil? If so, why not just switch to a thicker viscosity? Or, do you want the added protection of more antiwear additives? Once again, why not switch to an oil that has moly in it?
I really think those STP additives do more harm than good, as I bet they mess up the chemistry in the oil and cause it to deteriorate at a much faster rate.
What is that stuff supposed to do?
When I was a growing up the old-timers always used a can with every change. I never understood it other than being some "secret advantage" and haven't touched the stuff since I was 16. Guess I filed it in the snake-oil category.
Has anyone done an analysis of it, with or without oil?
Molakule did a nice write up on this awhile back,find his member number and do a search
In the old days,say a guy with 20 wt oil during winter in his car or truck that was near time to change the oil and just wanted either to pull a load or make a fairly long high speed road trip and drain when he returned it was effective in that sense,we must differntiate the STP's as the 4 cylinder treatment is somewhat thinner and contains more that just a thick bright stock and Zinc,Valvoline has a simaler product that has a graph showing it is superior to STP in oxidation ect,it should not be used in the differential. I use the 4cylinder additive to aid in breaking in cams with high spring pressures and hi intensity lobe designs"especially the unsymetrical lobes"
Oddly,Lucas Oil Treatment says much about using it in the crankcase,I think imo it would be better used in a gear box but I would not even do that imo. Only in a oil burning engine would I use any of these I have spoke about.
There are only two additves that are worth using imo and that is the thin Synergyn additive and the Shaeffer 132 Moly,,but would only use these in certain circumstances. I make my oil do it's job by it's self on daily drivers,if it needs some help,maybe a better oil or the same oil in a different viscosity that is up to the task should be used
I used STP once. When my Silver Hawk started burning 2 qts every 50 miles on an overnight crosscountry trip (Kansas to DC). 2 cans and I got to DC on 1 qt and sold the car.
Now that I know something about oil, I wouldn't touch the stuff. You don't want to mess with additive balance, either in the engine or dif. Worse yet in Transmissions where a lot of people put it and screw up their syncronizers. I just got back another set of analysis reports today and found out how a customer destroyed his Screw Compressor with an after-market additive.
I too had grouped STP in with the other automotive snake oils. Then I observed my son-in-law rebuilding a motor. Earns his living doing what he calls heavy repairs and rebuilds.
He was using STP to coat the parts as he put them together. When asked he said its not that STP is great, any thick oil would do. But STP comes in just the right size to complete a V-6/8 assembly.
I once saw an interview with Richard Petty, just after he won a race. After being asked how he ran so good that day, he replied "I had more STP in my engine than the other guys did!".
STP was his major sponsor.